Various flat screens have already been proposed, for example, there are so-called "active" screens such as electroluminescent screens or plasma panels, and there are so-called "passive" screens such as flat liquid crystal screens.
Generally speaking, such flat screens are constituted by two parallel plates or blades, at least one of which is transparent, and these plates or blades have respective coverings of mutually orthogonal electrodes, with electro-optical material being disposed therebetween.
The crosspoints of n line electrodes by m column electrodes constitute nm electro-optical cells whose state is controllable by the potentials applied to said electrodes.
FIG. 1 shows a matrix M of nm electro-optical cells referenced ce.sub.11 to ce.sub.nm defined by the crosspoints between n line electrodes referenced l.sub.1 to l.sub.n and m column electrodes referenced c.sub.1 to c.sub.m.
In spite of the considerable study and research effort which has been devoted to producing flat screens, few practical flat screens are commercially available at present.
The Japanese SEIKO corporation offers a liquid crystal matrix screen having the reference number FT 1616 in which trichromatic color is provided by means of a grid of optical filters.
The Japanese SHARP corporation, the Finnish LOHJA corporation and the American PLANAR SYSTEM corporation propose electroluminescent matrix screen modules of n lines by m columns using ZnS:Mn which emits yellow light. These matrix screens display alphanumeric and graphical information at a brightness of at least 50 cd/m.sup.2 and at a pixel pitch lying in the range 0.3 mm to 0.4 mm.
Pages 128 to 129 of the document SID 82 DIGEST (by R. E. Coovert et al.) describes two-color electroluminescent screens made by stacking two electroluminescent elements. The common electrode is made of a transparent and electrically conductive film of ITO (Indium Tin Oxide). Such screens are not sufficiently reliable. Making a film of ITO often requires heat treatment at 500.degree. C., and such temperatures damage the long-term performance of dielectric layers.
U.S. Pat. No. 4,155,030 (IFAY CHANG) describes a polychromatic screen implementing an internal memory phenomenon.